The present invention relates to an improved dielectric heating system having a simplified contact system to automatically connect the bottom electrode to ground.
Uses of dielectric heating/drying systems are known and are currently in use or have been proposed for use in agriculture, polymer manufacture, pharmaceuticals, bulk powder, food processing, wood products, panel manufacture, and other industries. One of the key industries using these dielectric heating/drying systems is the wood products industry and the present invention will be described particularly with respect to the wood products industry, although the invention, with suitable modifications where required, may be applied in the other industries in which dielectric heating/drying is to be performed.
In dielectric drying or heating systems particularly those used for drying wood, it is the conventional practice to load the material to be dried onto a wheeled cart and to roll the loaded cart into the kiln which is provided with rails to receive the wheels of the cart. See for example, U.S. Pat. No. 3,986,268 issued Oct. 19, 1976 to Koppelman and U.S. Pat. No. 4,472,618 issued Sep. 18, 1984 to Cloer. In these systems, the carts serve as both a conveyor and electrode. Clearly the cart, which is the electrode, is moveable and thus the cart-electrode must be moved into the kiln and connected electrically before the kiln chamber is closed and the drying process proceeds.
As above indicated, all of these cart systems require manually connecting the grounding system to the cart loaded with material to be dried and positioned in the kiln before the drying cycle may be started and disconnecting the grounding system after drying and before the loaded cart may be moved from the kiln. This loading and unloading, connecting and disconnecting etc., necessitates the use of professionally trained personnel both for safety and operating procedures to better ensure there are no major problems or accidents. These limitations imposed primarily by the use of carts have given the process of dielectric drying a reputation as being non-robust in that it requires flimsy attachments, which lead those in the lumber industry to imply that the technique is still in the research or experimental stage, and has not yet been developed for commercial industrial purposes.
U.S. Pat. No. 3,986,268 issued Oct. 19, 1976 to Koppelman recognized the problem of carts and in one embodiment employs vertical electrodes and uses a conveyor (roller conveyor) to deliver the load to be dried into position between the vertical electrodes and then after drying to convey the dried load from between the electrodes. This system could permit computer-controlled operation, however it was found that uniform contact of the vertical electrodes with the sides of the load was difficult and could not be consistently made whereby the effectiveness of the system was compromised.
U.S. Pat. No. 6,080,978 issued Jun. 27, 2000 teaches the use of a conveyor system with the conveyor portion within the kiln directly connected to ground to provide a continuous system where attachment of grounding straps to a cart is eliminated entirely. This system is more suited to applications requiring both an infeed and outfeed door where the load does not need to be reversed.
It is the main object of this invention to provide a cart conveying system for a dielectric drying system wherein the grounding of the cart is automatically accomplished by movement of the cart to operative position.
Broadly the present invention relates to a method and apparatus for dielectric drying of a load comprising positioning the load on a carrier incorporating a secondary electrode, moving the load on said carrier substantially horizontally into a chamber having a primary electrode positioned above said load when said load on said carrier is moved into an operative position in the chamber, elevating said carrier and said load to vertically move said load into heating position while simultaneously connecting said carrier and thereby said secondary electrode to ground, applying high frequency power to said primary electrode to dielectrically heat said load.
Preferably said high frequency is radio frequency (RF).
Preferably said elevating moves said load into intimate contact with said primary electrode.
Preferably after said heating is completed, said carrier and said load are lowered to said operative position and then said carrier and said load are removed from said chamber.
Preferably during drying, said load is separated from said primary electrode and weighed.
Broadly, the present invention relates to an apparatus for dielectric heating of a load comprising a carrier, means forming a secondary electrode on said carrier, a first set of electrical contacts along the outer periphery of said carrier and connected to said secondary electrode, a heating chamber, means defining a pathway for said carrier into said chamber, a primary electrode in said chamber positioned above said pathway, elevating means positioned along said pathway so that said carrier when positioned in operative position in said pathway may be lifted by said elevating means into heating position, a cooperating set of electrical contacts secured along said pathway, contacts of said set of cooperating contacts being in position to make electrical contact with adjacent contacts of said first set of contacts on said carrier and form mating connections between contacts of said first set of contacts and said cooperating set of contacts when said elevating means has raised said carrier to said heating position from said operative position, and means connecting said primary electrode to a source of high frequency power.
Preferably said mating connections are formed by a flexible contact and a plate contact.
Preferably said flexible contact is one of said set of cooperating contacts.
Preferably the chamber is provided with at least one door, and further electrical contact means are provided on said door, means to move said door to a closed position and to a sealing position wherein said door is sealed and said further electrical contact means are positioned to make electrical contact with an adjacent contact of said first set of electrical contacts when the carrier and load are elevated from operative position to heating position.
Preferably said flexible electrical contacts are made from fingerstock having fingers extending from a connecting band and formed into an open triangular cross section with a pair of spaced bearing areas forming one incomplete side of said triangular cross section, said connecting band forming one of said pair of bearing areas and free ends of said fingers remote from said connecting band forming the other of said pair of bearing areas.
Preferably said bearing area formed by the band is fixed to its supporting structure and the bearing area formed by the free end bears against but is free to move relative to the supporting structure.
Preferably contacts of said set of cooperating electrical contacts are provided on all sides and end of said pathway and cooperate with adjacent contacts of the first set of electrical contacts.
Preferably said carrier is formed by a plurality of carts in end to end relationship and with mating connection being formed between adjacent ends of adjacent of said carts.